Metaphorical, Meandering, and Emotionally Charged
If you're looking for something straightforward, easy-to-understand, and plot-driven, then An American Dream will drive you insane.
Mailer's most notable stylistic choice is his constant use of metaphors and similes—there are more of them in the novel than derrieres in a Nicki Minaj music video. This has an interesting effect, since we often spend more time reading metaphors than simple descriptions of a scene. For instance, Rojack says that a "moment of fright flew like a comet across the harbor of [his] calm" and "[his] eyes had the blue of a mirror held between the ocean and the sky" (2.17). As a result, An American Dream is driven far more by Rojack's emotional state than by anything resembling a plot.
Instead of a tidy series of cause-and-effect, the book uses a series of repeating symbols and irrational thought-processes to drive the story forward. After all, Rojack is an irrational guy—it would be impossible for him to tell a rational story.
The combination of these stylistic choices makes the book feel more like a dream than reality. And guess what? That's exactly how Rojack feels.